Re: DOS 3.3 Nibble Encoding

[ag471@yfn.ysu.edu (Eric S. Ford)]

In a previous article, mark@trillian.jsc.nasa.gov (Mark Manning) says:

>> There is absolutely nothing special about track 0, sector 0.
>
>I can not agree with the last statement.  Logic alone tells me that
>the boot track on a ProDos disk is the same as the boot track on a
>DOS 3.3, Apple Pascal, CP/M, or any other formatted disk for the Apple
>computer.  The rest of the disk may be formatted differently, but the
>boot track has to be standardized so the system will boot properly.
>Therefore, the boot track is special by definition.

The boot track "special-ness" discussion began with a question regarding the
difference between the old 13 sector disk format and the less-old 16 sector 
format.  The change in controller card ROM changed the way track 0 sector 0
is formatted _only_.  The rest of the disk may be formatted in a variety of
manners - there in lies the variability of copy protection schemes.  The ROM
code is only concerned with that first sector, once it is loaded it is given
control and then it determines what formatting must be present.  The changes
in the state machine which allow multiple 0's in the bit stream enable the
use of a bigger translation table and thus more efficient storage (i.e. 6-2
encoding versus 5-3).  But the translate table itself is first dynamically
built by the ROM in order to read T0, S0.  The disk data can either modify or
replace that table for subsequent sectors.


-- 
Eric S. Ford    ag471@yfn.ysu.edu